Method for Heat Treating a Product Contained in a Package Such as a Tray

ABSTRACT

Disclosed is a tray including at least one deformable wall. The product contained in the tray is heated, the vapour produced by the heating being allowed to escape through an opening in the pack. The product is heated, being taken to a desired temperature, a temperature-holding stage is carried out, the vapour being allowed to escape, a gaseous fluid is introduced into the pack at the end of this temperature-holding stage and the pack is closed totally.

This invention relates to a method for heat treatment of a product, in particular a food product, contained in a pack such as a tray having at least one deformable wall, in which the product contained in the pack is heated, vapour produced by the heating being allowed to escape through an opening in the pack.

A method of this type is already known through French patent application number 2 852 921.

Heating the product is useful for helping it keep. It may concern simple cooking—whether complete or partial—or alternatively concern pasteurisation or even sterilisation.

As is known, pasteurisation and sterilisation are used for destroying certain bacteria by heating, in order to push back the use by date of the product.

The consumable product is in particular a food product, such as vegetables, fruit or other food, whether fresh, blanched, pre-cooked or alternatively deep-frozen. It may also concern ready-cooked dishes including, for example, meat or fish or any food preparation.

It is known that, during the heating of a product of this type, vapour is produced. This is, in particular, water vapour, produced by the water which evaporates from the heated product due to the effect of temperature. In addition, the gas initially contained in the pack is expanded because of the rise in temperature, the excess gas volume forming part of the vapour which is allowed to escape from the opening in the pack at the time of heating to prevent the occurrence in the latter of overpressure which could lead to bursting or, in any case, to impairment of the pack and to failure of the heating method.

The above-mentioned French patent application particularly applies to a pack formed by a tray or by a soft pouch. According to this patent application, the opening in the pack, through which the vapour can escape, is formed by an area provisionally not welded between two walls of this pack and this opening is closed by welding at the end of heating.

At the end of heating, the pack is thus re-closed while its contents are still hot. During cooling, whether this is sudden or progressive, the volume of the gas contained in the pack decreases, which, as the pack wall is deformable, causes deformation of the latter. When the pack is formed by a soft pouch, this shrinkage may be an effect which is sought.

It is not so when the pack is formed by a tray, which must normally have a substantially parallelepipedic shape. Indeed, the consumer would be alarmed by the deformation of such a pack.

In order to try and prevent these deformations, European patent number 1 265 793 recommends finally re-closing the pack only when the temperature of the product it contains has reached a precise value and the inside of the pack still has overpressure so that the pack is not deformed after cooling. Indeed, the volume of gas at overpressure at the time of closure regains, after cooling, a volume which correctly fills the normal internal volume of the pack.

This method has a certain benefit but requires, on one the hand, monitoring the temperature of the product in the pack very accurately, which is extremely difficult on an industrial scale and, on the other hand, making sure that the opening in the pack is initially of dimensions such that a controlled overpressure will be ensured inside the pack at the time when the latter is re-closed.

Thus, this method has practical implementation difficulties.

The object of this invention is to propose a different solution to allow a release of vapour during heating and prevent excessive permanent deformation of the pack's deformable wall, arising from the heating.

This object is achieved due to the fact that the product is heated so as to be taken to a desired temperature, a temperature-holding stage is carried out, the vapour being allowed to escape, a gaseous fluid is introduced into the pack at the end of this temperature-holding stage and the pack is totally closed.

According to the invention, the heating comprises a temperature-holding or “temperature adjustment” stage, with which the desired cooking value (i.e. a value with which the desired cooking can be carried out), pasteurising value or sterilising value can be obtained and during which the vapour is allowed to escape. At the end of this stage, a gaseous fluid is introduced into the pack to cause an increase in its internal volume by means of a provisional deformation of its deformable wall and/or to cause increased overpressure. The pack can then be closed and, when the gas contained in this pack has regained its “normal” volume at ambient temperature, the pack will have regained a shape close to its initial shape, without excessive deformation. As will be seen below, the volume of gas introduced at the end of the temperature-holding stage can be determined in such a way that the final shape of the pack is substantially its initial shape, without appreciable deformation in comparison with it. This volume can also be determined in such a way that the final shape is intentionally slightly different from the initial shape. For example, the pack can be a semi-rigid tray closed by means of a soft film and the option can be taken to inject a slightly insufficient volume of gas so that, after cooling, the outside surface of the film is slightly concave, thus showing that the product has actually undergone heat treatment, while the tray walls retain their initial shape.

In other words, with the invention, the pack deformations can be prevented or controlled. The final shape is appreciated at ambient temperature and “normal” atmospheric pressure.

For the purposes of this invention, ambient temperature is the normal temperature for storing the products contained in the pack, i.e., for deep-frozen products, of the order of −18° C., for products stored in cold rooms, of the order of 4° C. and for products stored without cooling, of the order of 20° C.

Advantageously, the gaseous fluid introduced is at a temperature at least close to the temperature of the pack vapour during the temperature-holding stage.

In this way, a thermal shock between the contents of the pack and the gaseous fluid introduced into the latter is prevented. In addition, it is thus ensured that this gaseous fluid corresponds, from the point of view of its temperature, to the cooking, pasteurisation or sterilisation criteria sought. The temperature of this fluid can even be turned to account in order to assist pasteurisation or sterilisation.

Advantageously, the gaseous fluid is a neutral gas, such as nitrogen, a gaseous mixture of nitrogen/deoxygenate or air, advantageously filtered. This gaseous fluid is chosen depending on the products treated.

The introduction of the gaseous fluid is managed in such a way that, when the pack, once closed, regains ambient temperature, the pack wall is substantially free from deformation or deformed in a controlled manner. It is possible to proceed empirically, by introducing, in the same introduction conditions, various different volumes of gaseous fluids into various different packs and then determining therefrom which is the volume of fluid introduced which leads to the final shape desired, so as to decide on this volume as the one to be introduced in an industrial manner.

It is also possible to evaluate the volume of vapour which escapes from the pack during heating and temperature holding and to introduce a corresponding gaseous fluid volume into this pack. The evaluation of the volume of vapour which escapes can be carried out by direct measurement or, indirectly, depending on the length of time for which the vapour escapes and by taking into account the dimensions of the opening in the pack.

Then again, it is possible, from tests, to determine the volume of fluid to be injected depending on the deformation it causes in the pack wall.

The best conditions for introducing the gaseous fluid having thus been determined, particularly relating to the length of time for which this fluid is introduced or the quantity of fluid directly measured which is introduced, the method can then be industrialised.

To obtain controlled deformation, of limited amplitude and affecting only certain areas (film) of the pack, a quantity of gas slightly different from that which would be necessary to prevent deformation and advantageously slightly less than the latter can be injected to obtain controlled concave deformation. The difference measured in volume may be, for example, of the order of ±5% to ±10%.

The invention will be understood well and its advantages will appear better when the following detailed description, of an embodiment shown by way of example without limitation, is read. The description refers to the appended drawings, in which:

FIG. 1 is a vertical section view of a tray containing a product, prior to heat treatment,

FIGS. 2 and 3 are plan views of such a tray, for two variants for forming its opening,

FIG. 4 illustrates the stage of heating the product contained in the pack,

FIG. 5 illustrates the introduction of a gaseous fluid to the inside of the pack and

FIGS. 6 and 7 show the final closure of the pack, for the two variants in FIGS. 2 and 3.

The tray in FIG. 1 comprises a receptacle part 1 and a covering wall 2, such as a removable film, arranged across the opening of this receptacle. Food products such as, for example, peeled potatoes 3 are arranged in this tray. Typically, this tray is made of plastic or aluminium or a composite such as, for example, card/plastic, card/aluminium or alternatively aluminium/plastic and it has a relatively thin wall, this being, for example, 0.5 mm to 2 mm thick. The top wall of this tray is thus formed by the film 2, which is sealed on the tray edge formed by the receptacle rim 1A. The materials chosen for the receptacle and the film are of any types known for this type of tray, for example those based on aluminium or plastic, in particular polypropylene, possibly loaded as is recommended by patent application EP 1 313 651.

The film constituting the top wall 2 forms a wall which is easily deformable. However, in so far as the wall of the receptacle 1 is relatively thin, in reality all the tray walls are deformable but the film 2 is deformable to a greater extent.

Before the product 3 contained in the tray is subjected to heat treatment, this tray is closed in an incomplete way, an opening being left. In FIG. 2, this opening is formed by an interruption 4A in the sealing strip 4 between two walls of the pack, in the case in point between the receptacle rim 1A and the film 2.

In FIG. 3, the opening 5 is formed by a hole in one of the pack walls, in the case in point in the film 2, whereas the sealing strip 4 between the film 2 and the receptacle rim is continuous.

In a general way, the opening through which the vapour can escape at the time of sealing is of relatively small dimensions so that the inside of the pack is taken into overpressure at the time of heating, as this overpressure assists with the heating through of food products.

As indicated above, the vapour which is released comes, on the one hand, from the vaporisation of the water initially contained in the pack—particularly in the products which are arranged inside—and, on the other hand, from the expansion of the gas initially contained in the pack. For the purposes of this invention, these two sources of release gas are designated “vapour”.

In FIG. 4, the tray in FIGS. 1 and 2 is arranged in a heating enclosure 6. It is, in particular, a microwave tunnel or oven in which microwaves are emitted by a generator. The tray is carried by a support 8. In the case where the enclosure is formed by the internal space of a microwave tunnel, this support 8 can be a conveyor, the path of which goes through the inside of this tunnel, which has an entrance and exit for the tray, which are not shown. Of course, several trays can be in the tunnel at the same time, it being possible to adjust the length of the said tunnel and the pace of the conveyor travel in order that the duration of a tray's immobilisation in the enclosure is sufficient to allow the heating desired.

The tray and its contents are subjected to heating, on the one hand, to allow a rise in temperature of the product contained in the tray and, on the other hand, to carry out a temperature-holding stage. Conventionally, for pasteurisation, the temperature-holding stage is conducted so that the food product is kept at a temperature of the order of 80° C. to 100° C. for a given time, of the order of 3 to 5 minutes for example. For sterilisation, the product is taken to a temperature of the order of 120° C. to 130° C. for the necessary duration, of the order of 3 to 5 minutes for example. The heating and temperature-holding stages can be carried out in the same microwave enclosure. By way of a variant, two microwave tunnels, arranged end-to-end, can be used, the first one using higher power to carry out the temperature rise stage, whereas the second one only carries out the temperature-holding stage, with lower power.

Of course, heating by means of microwaves is only an example, it being known that other heating methods can be used, in particular heating by means of other electromagnetic waves such as high frequency waves or alternatively heating with steam.

The temperature-holding stage can be carried out as is shown in FIG. 4. It is indeed seen on the one hand that, under the effect of the overpressure generated inside the pack by the vapour produced during heating, its most deformable wall, in the case in point the film 2, has become deformed so as to increase the pack volume. The excess vapour V escapes through the opening 4A.

At the end of the temperature-holding stage, a gaseous fluid is introduced into the pack as is shown in FIG. 5. This introduction can be carried out in the heating enclosure as is shown in FIG. 5. However, it is appropriate to point out that it can also be carried out after the temperature-holding stage, when the pack comes out of the heating enclosure.

In the example in FIG. 5, this introduction of the fluid F is carried out through an injection nozzle 9 which is engaged in the opening 4A. This nozzle is connected to a gaseous fluid supply system represented symbolically by a square 10. It is appropriate to point out that the introduction of the nozzle into the opening 4A is easy as, under the effect of the release of vapour V, this opening is at least partially open, i.e. the film 2 is locally kept at a distance from the rim 1A.

The introduction of a nozzle is also easy, from an opening formed in one of the receptacle walls, like the opening 5 in FIG. 3. It is appropriate to point out that, because of the overpressure in the pack, the film 2 is therefore at a distance from the products contained in the pack, so that the introduction of the nozzle does not spoil these products.

However, instead of introducing a nozzle into the opening, it is possible to arrange a gas injection system in the immediate vicinity of this opening, whether this be the opening 4A or the opening 5, arrangements being made for the pressure and flow rate of the gaseous fluid to be sufficient for the latter to enter the pack.

As can be seen by comparing FIGS. 4 and 5, at the time of introducing the gaseous fluid F into the pack, the film 2 has become more deformed, increasing the pack internal volume further.

After the introduction of the gaseous fluid, the pack is closed totally. In FIG. 6, this closure is carried out by completing the welding strip 4, as indicated by the hatched area 4B. The opening 4A can, in particular, be made and re-closed as is recommended in French patent application 2 852 921.

In FIG. 7, this closure is carried out by closing the hole 5 by means of a cap 5′.

Advantageously, as indicated above, the gaseous fluid is introduced through the opening through which the vapour is allowed to escape during temperature holding. By way of a variant, another orifice can be used. In particular, the vapour can be introduced through a nozzle forming a needle which, for introducing the gaseous fluid, pierces one of the pack walls. The hole thus formed is then re-closed by, for example, arranging a cap across this hole; the opening through which the vapour V has escaped is also closed by sealing or by fitting a cap, for example.

Advantageously, the gaseous fluid which is introduced into the pack at the end of the temperature-holding stage is at a temperature between 80% and 130%, preferably between 90% and 110%, of the vapour temperature during the temperature-holding stage. As indicated above, choosing a gaseous fluid temperature at least close to the vapour temperature is advantageous. When even a higher temperature for this gaseous fluid is chosen, in particular when the latter temperature is close to 130% of the vapour temperature, the gaseous fluid thus heated can be used for assisting with the cooking, pasteurisation or sterilisation.

With tests carried out in various different conditions of introducing vapour the pack at the end of the temperature-holding stage, it is possible to determine what the optimum gaseous fluid introduction conditions are so that, once the pack has been closed and after its contents have cooled, the balance of pressures between inside and outside the pack is such that the walls of this pack remain substantially free from deformation compared with that which is shown by FIG. 1 or that they are only slightly deformed in the way desired, only the film 2, for example, being slightly concave.

The gaseous fluid introduction conditions are defined by parameters such as: the temperature of the gaseous fluid introduced, its composition, the volume of the fluid introduced or alternatively, for determining this volume indirectly, the duration of the introduction stage, the flow rate of fluid at the outlet from the injection system and entering the pack, . . . .

Tests can also be carried out for various different types of pack openings, it being known that the size of the opening obviously has an influence on the volume of vapour which escapes from the pack during the temperature-holding stage. This volume of vapour can be determined indirectly if the time for which the gaseous release occurs and the dimensions of the opening are known, in the knowledge that the pressure of this vapour is generally known from another source.

Thus, advantageously, a volume of gaseous fluid corresponding at least approximately to the volume of vapour which escapes from the pack during heating and temperature holding is introduced.

As indicated above, these volumes of vapour and gaseous fluid can be determined by tests, in an empirical manner and by comparison between various different tests.

It is possible, for example, to evaluate the volume which escapes and then to introduce a corresponding volume. It is also possible to go ahead with tests in the same conditions, introducing various different volumes of gaseous fluid and comparing the initial mass of the pack with its contents and the mass after closure and cooling which, by calculation, gives the difference between the volume of vapour which has escaped and the volume of gas introduced.

Advantageously, the volume of gaseous fluid introduced is determined in such a way that this volume is substantially equal to the volume of vapour which escapes from the pack during heating and temperature holding, preferably being lower than this volume. For example, the volume of gaseous fluid introduced into the pack can be of the order of 5% to 10% lower than the volume of vapour which has escaped from the said pack. With this, it can be arranged that, after the pack contents have cooled, the most deformable wall of the said pack, in particular the film 2 which forms the top wall, has a slight concavity, which assists with the stages of further handling of the pack, in particular the stacking of several packs of this type. This wall which is kept slightly concave can also be the bottom of the receptacle. This concavity is therefore slight enough not to constitute a deformation which is troublesome for the consumer but, as indicated above, it has certain advantages. For the purposes of this invention, this concavity is slight enough for the pack to be considered “substantially free from deformation” at the end of cooling.

The volume of gaseous fluid introduced into the pack and the volume of vapour which has escaped from it are compared in conditions which make this comparison possible, in particular adjusting these volumes, by calculation, to equivalent volumes at the same temperature, in particular the ambient temperature.

By way of a variant, the gaseous fluid is introduced into the pack, causing deformation of the deformable wall and the pack is closed when it is found that this deformation has reached a given level.

As has been indicated above, the introduction of the gaseous fluid can cause deformation of the film 2 (see FIG. 5). The option can be taken, by means of tests on several identical packs having the same contents and heated in the same conditions, to close the packs for different deformations of their walls 2 and then to determine the pack which has the most favourable geometry at the end of cooling of its contents. The deformation found for this pack will then be decided on as a criterion for industrialisation of the method. The level of the deformation is, in particular, reflected in the amplitude of the said deformation.

It has been indicated above that, at the time of heating, this wall also deforms but to a lesser extent. At the time of the temperature-holding stage, the flow rate of vapour escaping through the opening 4A or stabilises, with the result that the deformation of the film 2 also stabilises. The option can be taken to measure the level of deformation at which the pack is closed in a relative manner, by comparison between the deformation stabilised at the time of temperature holding and the deformation obtained just before closure, after introduction of the gaseous fluid.

Then again, the option can be taken to introduce the gaseous fluid into the pack for a predetermined length of time, at the end of which the pack is closed. Here again, with tests it is possible to check, taking account of introduction conditions (in particular pressure, flow rate and temperature) and determine which of these tests has led to the best final geometry of the pack. With this, it is possible to decide on a normal duration of introduction, which is used when the method is industrialised. 

1-13. (canceled)
 14. A method for heat treatment of a product, in particular a food product, contained in a pack having at least one deformable wall, wherein the product contained in the pack is heated so as to be taken to a desired temperature, a temperature-holding stage is carried out, vapour produced by the heating is allowed to escape through an opening in the pack during heating and temperature holding, a gaseous fluid is introduced into the pack at the end of said temperature-holding stage and the pack is totally closed.
 15. The method according to claim 14, wherein the gaseous fluid introduced is at a temperature at least close to a temperature of the pack vapour during the temperature stage.
 16. The method according to claim 15, wherein the temperature of the gaseous fluid introduced is between 80% and 130%, preferably between 90% and 110%, of the vapour temperature during the temperature-holding stage.
 17. The method according to claim 14, wherein a volume of gaseous fluid is introduced, corresponding at least approximately to a volume of vapour which escapes from the pack during heating and temperature holding.
 18. The method according to claim 17, wherein the volume of gaseous fluid introduced is determined in such a way that this volume is substantially equal to the said volume of vapour, preferably slightly lower than this volume, when the said volumes are adjusted to the same temperature.
 19. The method according to claim 14, wherein the gaseous fluid is introduced into the pack, causing deformation of the said deformable wall and the pack is closed when it is found that the said deformation has reached a given level.
 20. The method according to claim 14, wherein the gaseous fluid is introduced into the pack for a predetermined length of time, at the end of which the pack is closed.
 21. The method according to claim 14, wherein the gaseous fluid is a neutral gas.
 22. The method according to claim 14, wherein the gaseous fluid is air, advantageously filtered.
 23. The method according to claim 14, wherein the gaseous fluid is introduced through the said opening in the pack.
 24. The method according to claim 14, wherein the opening is formed by at least one interruption in a sealing strip between two walls of the pack and, at the end of the stage of introducing the gaseous fluid into the pack, the latter is closed by completion of this welding strip.
 25. The method according to claim 14, wherein the opening is formed by at least one hole in a pack wall and, at the end of the stage of introducing the gaseous fluid into the pack, this hole is closed by means of a cap.
 26. The method according to claim 14, wherein the pack is a tray made of plastic, the top wall of which is formed by a film sealed at the tray edge. 